As such, all it needs in addition is a pack that allows you to hold its tube-exit end at the bottom (gravity is what forces that refreshing liquid to your parched lips). You don't say what Gregory pack you have, but nowadays many of them have a zippered pocket specifically designed to hold the bladdered beast. Otherwise, just stick it in the pack near the top and near your back, then thread the hose out and route it to where you can clip it to your chest harness. A side pocket is another good place for a bladder. Really, there's no trick to it. Just make sure it's oriented so the tube exits out the bottom of the bladder.
As for the hydration pack, get the minimum model that meets your cycling needs. The CamelBak Classic ($40; www.camelbak.com) is a bare-bones model that carries a bladder and a few on-road essentials. If you need a little more packing room for clothing and so on, upgrade to their Lobo ($65). Gregory's Tread ($70; www.gregorypacks.com) is similar in size and utility to the Lobo. For either, simply remove the bladder and fit it into your backpack someplace where it'll work. Just don't throw it at any birds while doing soB